This invention relates to PEM fuel cells and more particularly to the reactant flow fields therefor.
Fuel cells have been proposed as a power source for many applications. One well known such fuel cell is the PEM (i.e., proton exchange membrane) fuel cell. PEM fuel cells include, in each cell thereof, a so-called xe2x80x9cmembrane-electrode-assemblyxe2x80x9d (hereafter MEA) comprising a thin (i.e., ca. 0.0015-0.007 inch), proton-conductive, polymeric, membrane-electrolyte having an anode electrode film (i.e., ca. 0.002 inch) formed on one face thereof, and a cathode electrode film (i.e., ca. 0.002 inch) formed on the opposite face thereof. Such membrane-electrolytes are well known in the art and are described in such as U.S. Pat. Nos. 5,272,017 and 3,134,697, as well as in the Journal of Power Sources, Volume 29 (1990) pages 367-387, inter alia. In general, such membrane-electrolytes are made from ion-exchange resins, and typically comprise a perfluoronated sulfonic acid polymer (e.g. NAFION(trademark) from DuPont). The anode and cathode films, on the other hand, typically comprise (1) finely divided carbon particles, very finely divided catalytic particles supported on the carbon particles, and proton conductive material (e.g., NAFION(trademark)) intermingled with the catalytic and carbon particles, or (2) catalytic particles, sans carbon, dispersed throughout a polytetrafluoroethylene (PTFE) binder. One such MEA and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993, and assigned to the assignee of the present invention.
The MEA is sandwiched between sheets of porous, gas-permeable, conductive material, known as a xe2x80x9cdiffusion layerxe2x80x9d, which press against the anode and cathode faces of the MEA and serve as (1) the primary current collectors for the anode and cathode, and (2) mechanical support for the MEA. Suitable such primary current collector sheets comprise carbon or graphite paper or cloth, fine mesh noble metal screen, and the like, through which the gas can move to contact the MEA, as is well known in the art.
The thusly formed sandwich is pressed between a pair of electrically conductive plates which serve as secondary current collectors for collecting the current from the primary current collectors and together define a single cell. A plurality of abutting cells define a fuel cell stack. The plates conduct current between adjacent cells (i.e., in the case of bipolar plates) internally of the stack, and externally of the stack in the case of monopolar plates at the ends of the stack. The secondary current collecting plates each contain at least one so-called xe2x80x9cflow fieldxe2x80x9d that distributes the fuel cell""s gaseous reactants (e.g., H2 and O2/air) over the surfaces of the anode and cathode. The flow field includes a plurality of lands which engage the primary current collector and define therebetween a plurality of flow channels through which the gaseous reactants flow between a supply manifold at one end of a flow channel and an exhaust manifold at the other end of the channel. Serpentine flow channels are known and connect the supply and exhaust manifolds only after having made a number of hairpin turns and switch backs such that each leg of each serpentine flow channel borders at least one other leg of the same serpentine flow channel (e.g., see U.S. Pat. No. 6,099,984).
The pressure drop between the supply manifold and the exhaust manifold is of considerable importance in designing a fuel cell. One of the ways of providing a desirable pressure drop is to vary the length of the flow channels extending between the supply and exhaust manifolds. Serpentine channels have been used heretofore to vary the length of the flow channels. Serpentine channels are designed to allow some limited trans-land gas movement between adjacent legs of the same channel via the diffusion layer so as to expose the MEA confronting the land separating the legs to reactant. In this regard, gas can flow from an upstream leg of the channel (i.e. where pressure is higher) to a downstream leg of the same channel (i.e. where gas pressure is lower) by moving through the diffusion layer over/under the land that separates the upstream leg from the downstream leg of the flow channel. However, when the legs of a channel are long, an excessive pressure drop can occur between adjacent legs of the same flow channel. Such excessive pressure drop can, in turn, result in the gaseous reactant excessively short circuiting between the adjacent legs, rather than flowing through the full length of the channel. Such trans-land movement of gas between adjacent legs is considered to be excessive when it exceeds the amount of reactant that can be reacted on the MEA confronting the land between the legs.
The present invention provides a flow channel configuration reduces short-circuiting of the reactant gases between adjacent legs of the same channel. The present invention overcomes the aforesaid trans-land short-circuiting problem by providing a flow field having flow channels with non-parallel legs the longitudinal centerlines of which converge/diverge from each other such that the centerlines of adjacent legs intersect each other outboard the channels. More specifically, the present invention is an improvement to PEM fuel cells of the general type which comprise (1) a proton exchange membrane having opposing cathode and anode faces on opposite sides thereof, (2) a gas-permeable, electrically-conductive cathode current collector engaging the cathode face, (3) a gas-permeable electrically-conductive anode current collector engaging the anode face, and (4) a current-collecting plate engaging at least one of the gas-permeable collectors and defining a gas flow field that confronts that gas-permeable collector. The improvement is applicable to flow fields that comprise a plurality of lands that engage the current collector and define a plurality of gas flow channels, each of which comprises at least one serpentine sector flow-communicating a gas supply manifold with a gas exhaust manifold. Each sector has: an inlet leg for receiving gas into the sector at a first pressure; an exit leg for exhausting from said sector at a second pressure that is less than the first pressure; and at least one medial leg intermediate the inlet and exit legs. Several medial legs may be used to increase the pressure drop from one end of a channel to the other. The inlet, exit and medial leg(s) for each channel are separated by a tapering land so that each leg has a longitudinal centerline that intersects the longitudinal centerline of a next adjacent leg of the same channel. A reverse turn (e.g. hairpin curve) in the channel at each end of the medial leg(s) connects the medial leg(s) to adjacent legs of the same channel. Flow fields in accordance with the present invention may include a single such sector, or a plurality of such sectors serially arranged in the general direction extending between the supply and exhaust manifolds.